They are like large table runners, only for a bed. It allows you to make a pretty quilted accent for your bed without the commitment to making the larger bed-size quilt. Many display them across the foot of the bed as an accent to an existing quilt or bedspread or if you have a plain spread, the topper can be the focus.
I have several ideas in mind for making one for our bed. It would probably be an accent on top of the duvet I currently have on there.
Here are some examples: